Machining apparatus

ABSTRACT

A machining apparatus for conducting at least one of a cutting process and a grinding process, comprises a stationary base; and a working device mounted on the stationary base and having a degree of freedom of more than two axes to conduct at least one of a cutting process and a grinding process, the working device comprising a first working bench which is formed of a ceramic material and is movable linearly with a degree of freedom of not less than one axes or more while holding a work piece or a tool.

BACKGROUND OF THE INVENTION

This invention relates to a machining apparatus having a degree offreedom of machining of three axes or greater.

Herein, the term “axes” or “an axis” is defined as a portion of amachining apparatus that functions as the reference line or plane of amovement for machining, for example, the rotary shaft of a swivel table,the slideway of a slide table, etc. The degree of freedom of movementfor machining is one for each of the “axes”.

A machining apparatus for applying cutting machining, grindingmachining, or the like to an optical element and a die for the formationthereof is known (refer to the patent literature 1).

[Patent literature 1] The publication of the unexamined patentapplication 2003-39294.

In this connection, in order to generate a complex curved surfacecorresponding to an aspherical surface of an optical element with a highaccuracy, a degree of freedom of machining of two or less isinsufficient, and a degree of freedom of machining of three or greateris required. However, in a machining apparatus having a degree offreedom of machining of three axes or greater for the generating of anoptical surface which is a free surface, whichever of cutting orgrinding the machining may be, it becomes necessary, a shaping processin which a machined surface is generated while a number of throughmachining steps are being connected one after another; therefore, it hasa feature such that it requires a very long time, which is not found inthe machining by means of any other kind of machining apparatus. Forthat reason, in order to carry out a machining for generating a freesurface with a high accuracy and a high efficiency, it is necessary toactualize the following necessary conditions more sufficiently than aconventional machining apparatus.

[Necessary Conditions for Actualizing High Accuracy]

(1) There is a requirement for making higher the control accuracy of themachine, and for that purpose, it is necessary to control a workbench (aslide table or a swivel table) driven by a high-speed servomechanismwith a high accuracy achieved by the improvement of the resolving powerof the position measurement of each of the axes, or to lighten thedriven workbench by the use of a material having a small specific weightso that it may comply with the drive instruction of the servomotor at ahigh speed.

(2) Because the machine has a block structure with each of the axisportions placed over one another in two or three stages, which makes theoverall stiffness tend to become lowered for the structural reason, itis necessary to make higher the stiffness of each of the axis portionsas much as possible, and for that purpose, it becomes important to lowerthe viscosity of the pressure transmission medium of the static-pressureslide, or to use a material having a high Young's modulus.

(3) It is also very important to make the machine not subject to thetemperature variation of the environment, and in a case where astatic-pressure slide is used, it is important to lower the viscosity ofthe pressure transmission medium in order to suppress a useless heatgeneration at the static-pressure surface, and also it is important toselect a material having a small coefficient of linear expansion foreach of the members.

(4) It is necessary to shorten the machining time for the purpose ofmaking it possible to finish the machining before the environmentvariation becomes large and carrying out the machining with a highefficiency; therefore, the following things are necessary: to complywith the high resolution of the position measurement by making higherthe speed of the servomechanism in order to drive the axis at a highspeed, and to lower the viscosity of the pressure transmission medium sothat the lowering of stiffness of the static-pressure surface and avibration may not be generated.

(5) In order to prevent the influence of the vibration due to thevibration of the floor and the oscillation of the machine body itself,it is necessary, an active vibration-reducing table or the like whichpractices a control in such a way that it detects and suppresses avibration in an active manner.

However, heretofore, it has never existed a machining apparatus having adegree of freedom of machining of three axes or greater that can achievethe compatibility of a high accuracy of an order of sub-micron with ahigh efficiency of machining.

SUMMARY OF THE INVENTION

This invention has been made in view of the above-mentioned points ofproblem in conventional technologies, and it is an object of thisinvention to provide a machining apparatus having a degree of freedom ofmachining of three axes or greater which is capable of achieving thecompatibility of a high accuracy with a high efficiency of machining.

A machining apparatus as set forth in Item (1) is a machining apparatusfor practicing a cutting machining or a grinding machining with a degreeof freedom of three axes or greater, wherein a first workbench holding awork piece or a tool and being linearly movable with a degree of freedomof at least one or greater is formed of a ceramic material.

In a high-accuracy machining apparatus having a measurement resolutionof 10 nm or less for the axis position based on a conventionaltechnology, the material of the workbench to be driven has been carbonsteel such as cast iron or S45C in most cases, and its specific weightis about 7.8 g/cm³. Therefore, because the workbench becomes heavy, andthe power of a servomotor required for driving this becomes high, therehas been a problem that the quantity of heat generation from this motorbecomes large. Further, because it has not been possible to move theworkbench at a high speed, there has been also a problem that the speedof machining becomes slow, which makes the machining time longer, and ifa variation of the environment such as the temperature occurs, thedeformation of the workbench due to thermal expansion and the drift ofposition of a work piece and a tool become large, which makes ahigh-accuracy machining difficult.

Further, in a machining apparatus having a degree of freedom ofmachining of three axes or greater, it is carried out in most cases, ashaping machining, that is, a machining in which, in association with areciprocating movement with respect to one axis, another movement withrespect to another axis is carried out, which moves a tool along thecross-sectional shape of a free surface, and by the three-dimensionalaccumulation of a number of movements of the tool one above another, acurved surface is generated by cutting, grinding, etc.; however, thisshaping machining takes a very long time, particularly, in generating ahigh-accuracy optical surface, even for a work piece of a size of anorder of several centimeters, it is general that a machining time ofseveral ten hours is required for it. Accordingly, it is very importantfor the purpose of carrying out a high-accuracy generating of an opticalsurface, how to make the machining difficult to undergo the influence ofthe change of the environment such as the temperature during the periodof time of machining, or to make the variation of the environment duringthe machining smaller by shortening the machining time.

In contrast with this, according to this invention, even in a case wherea temperature variation occurs, because the thermal expansioncoefficient can be suppressed at a low value in comparison with a steelor the like, by forming the above-mentioned first machining table of aceramic material, the accuracy of machining can be maintained, while themachining time can be shortened because the acceleration anddeceleration ability is raised by the workbench being madelight-weighted. Further, because it is enough if the power of the motoror the like as an axis member driving means for driving theabove-mentioned first workbench is low, energy saving can be achievedand also the heat generation can be suppressed; therefore, ahigher-accuracy machining can be actualized.

Further, in a machining apparatus having a degree of freedom ofmachining of three axes or more, it is essential to take a structuresuch that a workbench of one axis is placed on a workbench of anotheraxis because the number of axes are large, and usually, a multi-axisstructure is made up of two or three stage workbenches being piled. Thismeans, in other words, that the stiffness of each axis member isaccumulated to come to support a tool or a work finally; therefore, itindicates that, in comparison with a usual machining apparatus havingtwo axes or less, its axis stiffness has to be made two or three timeshigher. On top of it, for a tool or a work piece in order to avoid theinterference between the movements with respect to their respectiveaxes, it is necessary for a tool or a work piece to be fixed at a placedeviated remarkably from the slideway of each of the axis members in anoverhanging manner; therefore, because the machining force acts on theslideway as a moment of force during machining, and further, themachining is to be carried out in a condition that the stiffness is madeweaker, it is necessary to make the stiffness of each of the axismembers as high as possible in the case of a high-accuracy machining ofthree axes or more.

In contrast with this, according to this invention, by forming theabove-mentioned first workbench of a ceramic material, in comparison,for example, with a conventional workbench made of cast iron, its weightcan be reduced by a large margin and its Young's modulus can be made twotimes larger; therefore, bending caused by the self-weight can besuppressed, and it is obtained a structure such that a resonance or thelike is hard to produce because the specific frequency of the vibrationis made higher.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a five-axis machining apparatus 10 ofthis invention;

FIG. 2 is a cross-sectional view of a slide table and a swivel table ina modified example of this embodiment;

FIG. 3 is a cross-sectional view of a slide table and a swivel table inanother modified example of this embodiment; and

FIG. 4 is a cross-sectional view of a slide table in further anothermodified example of this embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Firstly, the preferable structures according to the present inventionare explained.

A machining apparatus set forth in Item (2) is a machining apparatus asset forth in the Item (1), wherein the aforesaid ceramic material has acoefficient of linear expansion of 5×10⁻⁶ K⁻¹ or less; therefore, ascompared to cast iron, its thermal expansion can be suppressed to be lowby half or less; therefore, the accuracy of machining can be maintainedsecured while the acceleration ability is made higher owing to itsweight being made lighter, which makes it possible to shorten themachining time.

A machining apparatus set forth in Item (3) is a machining apparatus asset forth in the Item (1) or (2), wherein the aforesaid ceramic materialcontains silicon nitride of 50% or more by weight as converted intoSi₃N₄ and has a specific weight of 4 g/cm³; therefore, the weight of theaforesaid workbench becomes a half or less of a conventional one, andeven if the power of the axis member driving means for driving it (forexample, a motor) is the same, the acceleration can be made two timestheoretically. Accordingly, the time to reach the target machining speedis reduced by half, and the quantity of heat generation is also halvedfor that reason. Further, because it becomes fast the response of theservomechanism for detecting the position of the aforesaid firstworkbench and applying a feedback to the driving motor, the accuracy ofmovement and the accuracy of positioning of said first workbench aremade higher. From this viewpoint, the specific weight may be better ifit is smaller; however, because a high Young's modulus is also necessaryat the same time for the purpose of raising the stiffness, the materialshould be selected with it taken into consideration.

A machining apparatus set forth in Item (4) is a machining apparatus asset forth in any one of the Items (1) to (3), wherein the aforesaidceramic material has a Young's modulus of 200 GPa or greater; therefore,as compared to cast iron, Young's modulus can be made greater by 30% ormore, and for this reason, it is possible to improve the stiffness ofthe aforesaid first workbench.

The main part of the material of a conventional machining apparatus iscast iron, and its Young's modulus is as low as an order of 150 GPa;therefore, if a combination of axis members having a degree of freedomof three or greater totally is to be used, it is impossible to achieve ahigh-accuracy machining. Further, the coefficient of linear expansion istoo high as 10×10⁻⁶ K⁻¹, and in a condition that the environmentaltemperature varies during a machining of a long period of time, thermalexpansion or contraction is generated to make the machining point drift.As shown in Table 1, in materials to be considered as a material formachine structure each, Young's moduli and coefficients of linearexpansion are collected; ceramic materials such as silicon nitride,sialon, and silicon carbide become a candidate for the material to beused in a machining apparatus. Among these ceramic materials, sialon isa mix material composed mainly of silicon nitride with alumina etc.contained, and the basic property is similar to silicon nitride as itsmain component. Accordingly, both the materials have a high breakagetoughness each as a ceramic material, and have an advantage that theycannot be broken easily. Therefore, in particular, a ceramic materialcomposed mainly of silicon nitride satisfies the condition of having aspecific weight of 4 g/cm³ or less, a coefficient of linear expansion of5×10⁻⁵ K⁻¹ or less, and a Young's modulus of 200 GPa or greater, and atthe same time, it is conspicuous in that it has a high breakagetoughness and is hard to break; by using this material in a machiningapparatus of a degree of freedom of machining of three axes or greater,it is possible to stably practice an extremely high-accuracy machining.TABLE 1 Cast Nobinite Silicon iron Granite Invar CS-5 Al₂O₃ Si₃N₄ Sialoncarbode Specific 7.3 26 8.1 7.5 3.9 3.3 3.3 3.1 weight (g/cm³) Hardness200 500 190 160 1800 1400 1580 2200 (Hv) Breakage — — — — 4.0 6.0 6.03.5 toughness (MN/m^(3/2)) Young's 150 70 130 130 382 284 294 412modulus (GPa) Coefficient 11 7.0 1.5 2 7.2 2.7 3.0 4.0 of linearexpansion (× 10⁻⁶ K⁻¹) Thermal 41 18 11 13 29 13 21 83 conductivity(W/mK)

A machining apparatus set forth in Item (5) is a machining apparatus asset forth in any one of the Items (1) to (4), wherein the aforesaidfirst workbench is driven along a static-pressure slide by an axismember driving means with a frequency of 50 Hz (desirably 100 Hz) orhigher of a servo gain becoming −3 dB. In particular, even in a casewhere the moving velocity of the aforesaid first workbench is madehigher in order to raise the efficiency of machining, because the weightof said first workbench is made light-weighted, the compliance of saidfirst workbench can be maintained, even though the above-mentioned axismember driving means is subject to a control of such a high responsecharacteristic; therefore, it is possible to make the improvement of theefficiency of machining compatible with the high accuracy of themachining.

A machining apparatus set forth in Item (6) is a machining apparatus asset forth in the Item (5), wherein the aforesaid axis member drivingmeans is a linear motor. In a linear driving of a general machiningapparatus, a ball screw, a static pressure screw, or the like is alsoused, and in particular, in a machining apparatus for carrying outcutting machining or a grinding machining with a degree of freedom ofthree axes or greater like this invention, it is desirable to drive theaxis members at a high speed for the purpose of shortening the time ofmachining and achieving a high accuracy. However, as regards a ballscrew, a nut is supported through a number of balls by a screw, andowing to it, feeding of the axis member is carried out through therolling of balls in contact with the screw; therefore, the higher thespeed becomes, the stronger vibration is generated, which makes itdifficult to carry out a high-accuracy machining. Further, as regards astatic-pressure screw, although it is advantageous in that it does notbecome a source of vibration because the screw is supported by a staticpressure of oil against its nut thread, but the viscosity of the oil ismade high in order to keep the stiffness in the feed direction high,which makes a large torque necessary for the rotation of the screw at ahigh speed; thus, there are a problem of heat generation, a problem thatthe power of driving motor has to be made high, etc. From theseviewpoints, it can be said that it is desirable to use a linear motorwhich can actualize a high acceleration and a non-contact feeding. Ontop of it, in a machining apparatus of this structure, because theabove-mentioned first workbench to be driven is made of a ceramicmaterial, which is a non-conductive and non-magnetic material, theleakage magnetic flux due to the linear motor being placed to theabove-mentioned first workbench and the slide is very weak, which isdifferent from the case of a conventional metallic material such as castiron, and also an eddy current and an electric motive force due to themovement of the above-mentioned first workbench are not generated;therefore, the first workbench is very suitable for the use of such alinear motor for the reason that a high-accuracy and high-speed feedingwith a small amount of noise can be actualized.

A machining apparatus set forth in Item (7) is a machining apparatus asset forth in any one of the Items (1) to (6) further comprising ameasurement means having a resolution of 10 nm or less for measuring theposition of the aforesaid first workbench; therefore, the response ofthe servomechanism which detects the position of the above-mentionedfirst workbench to give a feedback to the axis member driving means ismade faster, which makes higher the accuracy of movement and theaccuracy of positioning of the above-mentioned first workbench.

A machining apparatus set forth in Item (8) is a machining apparatus asset forth in any one of the Items (1) to (7), wherein the machiningspeed of a work piece or a tool held by the aforesaid first workbench is600 mm/min or higher.

In a generating machining by means of a machining apparatus of a degreeof freedom of three axes or greater, as described before, becauseshaping machining for generating a free curved surface through theaccumulation of a cross-sectional shape formed by a reciprocatingmovement is mostly used, the machining time tends to become long. Forthat reason, it can be said that it is important to make the machiningtime shorter by making the machining speed higher, in order that notonly the efficiency of machining may be improved but also ahigh-accuracy machining may be actualized by the reduction oftemperature variation during the machining.

In the above description, it is the frequency characteristic of themeasuring sensor such as a laser scale for carrying out the measurementof the position of the axis member mainly that determines the upperlimit of the machining speed. The higher the accuracy of measurement ofa sensor to be used is made, the higher the accuracy of the positionsignal to be outputted becomes; however, because a number ofdisplacement signals are outputted even by minute displacements of anaxis member owing to it, its output frequency reaches several MHz easilyin a case where an axis member is moved at a high speed. For thatreason, the frequency exceeds the allowed frequency range of the analogamplification circuit in the measuring sensor and the wiring to theservo-driver, and the action of the axis member cannot comply with it,which produces an error.

Assuming that the signals are outputted by 1 nm unit in order to makehigh the accuracy of the detection resolution of the measuring sensor,the frequency becomes 10 MHz at the axis member speed 600 mm/min. Afrequency of this order is, in the present situation, the maximumfrequency value of a signal to be outputted by a measuring sensor withthe accuracy maintained. Accordingly, in order to practice a machiningat a high accuracy with a high efficiency in a short time, it isdesirable to practice a machining at this highest speed possible in thepresent situation, and from now on, if the frequency range of themeasuring sensor is improved, it is desirable to raise the speed ofmachining corresponding to the amount of improvement. Accordingly, asthis structure, it is most desirable from the viewpoint of theefficiency and accuracy of machining to make the speed of machining 600mm/min or higher with an expectation of some improvement from now onincluded.

Further, in order to control axis members with a high accuracy incompatibility with a high-speed driving, it is necessary that theservo-control capability can also respond to this frequency range of themeasuring sensor sufficiently, however, there is an actual situationthat the frequency range of the servo-loop is usually of an order of 50Hz to 100 Hz, or of an order of 200 Hz at the highest, which isremarkably low compared to the frequency range of the measuring sensordescribed above. Because an electrical circuit for carrying out thisservo-control has usually a frequency range close to several hundredkHz, it is understood that the response speed of this portion hardlyinfluences the frequency range of the servo-loop. That is, it is mostlythe mechanical response that determines the frequency range of theservo-loop, and the response depends mostly on the delay of timerequired for moving the above-mentioned first workbench or a secondworkbench from the time an instruction signal is converted into adriving force by the driving motor which is transmitted to theworkbench. Accordingly, it can be understood how much it is effectivefor actualizing a high-accuracy machining in compatibility with ahigh-speed performance that, as this structure, the mass of theabove-mentioned first workbench is lowered, which makes the force ofinertia and the moment of inertia weakened, and the mechanical responseis improved.

A machining apparatus set forth in Item (9) is a machining apparatus asset forth in the Item (8), wherein the aforesaid first workbench isdriven at the highest speed in said machining apparatus; therefore, itis possible to exhibit the above-mentioned effect more.

A machining apparatus set forth in Item (10) is a machining apparatuswhich practices a cutting machining or a grinding machining with adegree of freedom of three axes or greater, wherein a second workbenchwhich holds a work piece or a tool and is capable of swiveling with adegree of freedom of at least one axis or greater is made of a ceramicmaterial.

The material for making up a conventional swiveling workbench is castiron or steel in most cases. For that reason, the swiveling workbenchbecomes heavy, and the power of a servomotor required for driving thisbecomes high, which results in a large quantity of heat generation fromthe axis member driving means (for example, a motor). Further, becauseit is not possible to move the swiveling workbench at a high speed, themachining speed becomes slow, and the machining time becomes longer,which makes the variation of the environment such as the temperaturelarger; therefore, the deformation of the workbench due to thermalexpansion and the drift of position of a tool and a work piece becomelarge, which makes a high-accuracy machining difficult.

In contrast with this, by this structure, because it is possible tosuppress thermal expansion to be lower as compared to steel by formingthe aforesaid second workbench of a ceramic material, even in a casewhere a temperature variation occurs, the accuracy of machining can bemaintained and also the acceleration/deceleration performance is madehigher by the workbench being made light-weighted, which makes itpossible to shorten the machining time. Further, because it is enougheven if the capacity of a motor or the like as a driving means fordriving the above-mentioned second workbench is small, energy saving canbe achieved, and at the same time, the quantity of heat generation canbe suppressed; therefore, a higher-accuracy machining can be actualized.

A machining apparatus set forth in Item (11) is a machining apparatus asset forth in the Item (10), wherein the aforesaid ceramic material has acoefficient of linear expansion of 5×10⁻⁶ K⁻¹ or less. The effect ofthis structure is the same as the structure set forth in the Item (2).

A machining apparatus set forth in Item (12) is a machining apparatus asset forth in the Item (10) or (11), wherein the aforesaid ceramicmaterial contains silicon nitride of 50% or more by weight as convertedinto Si₃O₄ and has a specific weight of 4 g/cm³. The effect of thisstructure is the same as the structure set forth in the Item (3).

A machining apparatus set forth in the Item (13) is a machiningapparatus as set forth in any one of the Items (10) to (12), wherein theaforesaid ceramic material has a Young's modulus of 200 GPa or greater.The effect of this structure is the same as the structure set forth inthe Item (4).

A machining apparatus set forth in Item (14) is a machining apparatus asset forth in any one of the Items (10) to (13), wherein the aforesaidsecond workbench is driven along a static-pressure slide by an axismember driving means with a frequency of 50 Hz (desirably 100 Hz) orhigher of a servo gain becoming −3 dB. The effect of this structure isthe same as the structure set forth in the Item (5).

A machining apparatus set forth in Item (15) is a machining apparatus asset forth in the Item (14), wherein the aforesaid axis member drivingmeans is an AC servomotor. As regards the second workbench capable ofswiveling, its driving for swiveling is carried out by a DC servomotoror a method such that the resolution of the rotation angle is increasedby a gear provided in between; however, because there is a brush in a DCservomotor, owing to the variation of its frictional force and contactresistance, it is difficult to stably maintain a high-accuracy swivelangle. In a case where a speed reducing gear such as a worm gear or aharmonic gear is used, it seems that the angular resolution is improvedby an amount corresponding to the magnification of the speed reductionratio apparently, but actually, a stick-slip phenomenon occurs due tothe static frictional force of the gear, or a backlash is generated bythe poor meshing of the gears with one another, which makes a control ofa minute rotational angle difficult on the contrary. Further, thesemethods in which an external motor is mounted to a swivel shaft producenecessarily an eccentricity in the coupling of the motor shaft and therotary shaft, which makes the controlled rotational angle and the actualrotational angle not agree with one another in a strict sense, and thetorque stiffness is lowered by a flexible coupling member which is usedfor the coupling between both the shafts in order to ease the twist dueto the eccentricity and the unevenness of torque, which lowers thefrequency range of the feedback control to degrade the servo-controlcharacteristic; therefore, it does not always agree with the purpose ofthis structure for actualizing a high-accuracy machining.

In that point, a direct AC servomotor having a permanent magnet fitteddirectly at the swivel shaft is not provided with a frictional membersuch as a brush, and is capable of generating a torque directly in therotary shaft in a non-contact manner, which raises its stiffness also;therefore, if it is used in a machining apparatus having a degree offreedom of three axes or greater as this structure, it is desirable forthe practice of a high-accuracy machining. On the other hand, heat isgenerated in the portion of the coil built in an AC servomotor; however,if the swiveling second workbench is made of a ceramic material as thisstructure, the temperature drift of the position of a tool or a workpiece on the second workbench is small, and heat is hard to transfer tothe first workbench which carries this second workbench thereon, whichmakes it difficult to bring about a thermal expansion or contraction,because the ceramic material such as silicon nitride or sialon has avery low thermal conductivity; this is very advantageous. That is, aproblem in using an AC servomotor is solved successfully by thisstructure; therefore, it can be said that it is appropriate to use an ACservomotor for the driving of the swiveling second workbench.

A machining apparatus set forth in Item (16) is a machining apparatus asset forth in any one of the Items (10) to (15) further comprising ameasuring means having a resolution of a 1 angular second or less formeasuring the angle of the aforesaid second workbench. The effect ofthis structure is the same as the structure set forth in the Item (7).

A machining apparatus set forth in Item (17) is a machining apparatus asset forth in any one of the Items (10) to (16), wherein the rotationalmachining speed of a work piece or a tool held by the aforesaid secondworkbench is 1°/sec or higher. The effect of this structure is the sameas the structure set forth in the Item (8).

A machining apparatus set forth in Item (18) is a machining apparatus asset forth in the Item (17), wherein the aforesaid second workbench isdriven at the highest speed in said machining apparatus. The effect ofthis structure is the same as the structure set forth in the Item (9).

A machining apparatus set forth in Item (19) is a machining apparatus asset forth in any one of the Items (10) to (18), wherein a support tablefor supporting the aforesaid second workbench is formed of at least oneof a ceramic material satisfying at least one of the conditions ofhaving a coefficient of linear expansion of 5×10⁻⁶ K⁻¹ or less,containing silicon nitride of 50% by weight as converted into Si₃N₄ andhaving a specific weight of 4 g/cm³ or less, and having a Young'smodulus of 200 GPa or greater, and an alloy containing nickel from 10%by weight to 50% by weight. In particular, in the case where theaforesaid second workbench is carried on the aforesaid first workbench,it is desirable that said support table is formed of an alloy containingnickel from 10% by weight to 50% by weight (for example, invar, incolloy904, or the like).

A machining apparatus set forth in Item (20) is a machining apparatus asset forth in any one of the Items (5) to (9) and (14) to (19), whereinat least one of the aforesaid static-pressure slide and the base forfixing it is formed of a ceramic material satisfying at least one of theconditions of having a coefficient of linear expansion of 5×10⁻⁶ K⁻¹ orless, containing silicon nitride of 50% by weight as converted intoSi₃N₄ and having a specific weight of 4 g/cm³ or less, and having aYoung's modulus of 200 GPa or greater.

The shaping machining by means of a machining apparatus of a degree offreedom of machining of three axes or greater requires a very long time,and on top of it, because the axis members are placed in two or threestages one over another, actually, there is a problem that the overhangof mounting of a tool or a work piece becomes large through the objectslying between the base and the point of machining such as the tool restand the workbench, and in a case where the workbench and thestatic-pressure slide is thermally expanded or contracted by atemperature variation, the relative position between a tool and a workpiece is not only shifted simply in one direction, but it exhibits alsoa simultaneous generation of an oscillation like a moment. Further,because the temperature cannot be made uniform from the bottom to thetop of the work benches piled up high, in each of the axis members piledup, the workbench is subjected to a thermal expansion or contraction,and in response to it, the final machining point of the tool and theposition of work piece are displaced in a complex manner. Therefore, itis almost impossible to correct this machining displacement (drift) dueto the temperature variation, and it can be said that, in order toprevent this, first, the thermal expansion/contraction itself shouldthoroughly be made small.

However, if the workbenches and the static slides are made of cast ironor steel as a conventional machining apparatus, it is extremelydifficult to prevent such an influence of thermal expansion orcontraction; therefore, in order to maintain a high temperaturestability, it is desirable to make the coefficient of linear expansionof the material of the first and second workbenches and thestatic-pressure slides 5×10⁻⁶ K⁻¹ or less. As understood from Table 1,this numerical value is equivalent to a half of the coefficient oflinear expansion of cast iron or steel, and by this condition, thermalexpansion is halved in each of the horizontal direction and the verticaldirection; therefore, the three-dimensional variation range of themachining point is reduced to ⅛, and a large effect can be obtained inthe stability against a temperature variation.

Further, as another large factor of the drift of the machining point,there is an expansion or contraction of the bases for fixing theirrespective axis members. Because the bases are large-sized, their heatcapacities are also large; owing to it, if a temperature difference isproduced locally, no equilibrium is established, and heat is continue tobe transferred slowly for an indefinite time, which keeps the localtemperature difference left as produced. From such a characteristic, ifthe temperature of a portion of the base is changed by machining, aphenomenon that the portion is thermally expanded or contracted toproduce a variation, which does not come to an end for an indefinitetime. For example, if a drop of cooling oil for the machining point usedin a cutting or grinding machining falls on the base, the temperature ofthat portion is varied to cause the base to start a variation. However,so long as the cooling oil continues to drop (so long as the machiningcontinues), the temperature of that portion continues to vary, andbecause it takes a very long time for the temperature to diffuse to thewhole of the base to establish an equilibrium state, the basetemperature continues to vary during that period of time. In the case ofa base, because its heat capacity is large, in a case where such atemperature drift is generated, an axis member which is fixed to thebase moves to one direction only, a movement like a moment is notgenerated, and there is almost a parallel movement only; therefore, ifthe tendency of the movement of an axis member is grasped, thecorrection for it is not impossible. However, because such a drift whichcontinues to be generated for several tens of hours becomes a largedisplacement, the residual difference that has not be corrected alsobecomes large. In this case too, it is understood that it is veryeffective not to take a countermeasure against the thermal expansion orcontraction generated but to reduce the thermal expansion or contractionitself. Accordingly, as regards the material for making up the basealso, by using not cast iron or granite as heretofore, but a materialhaving a coefficient of linear expansion of an order of half that ofthose, the temperature drift of the machining point can be reduced byhalf. In this case, a temperature drift of one direction which does notinfluence so much in the height direction but is large in the directionparallel to the base can be reduced, and a high-accuracy free curvedoptical surface can be generated.

A machining apparatus set forth in Item (17) is a machining apparatus asset forth in the Items (1) to (16), wherein at least one of theaforesaid first workbench, the aforesaid second workbench, the aforesaidstatic-pressure slide, and the bases for fixing them is formed of amaterial having a Young's modulus of 200 GPa or greater.

The material of conventional workbenches and static-pressure slides havebeen centered on cast iron, and their Young's modulus has been of anorder of 150 GPa. This means that when a load of 1000 N percross-section of 1 cm² is applied to any one of them, a member having alength of 10 cm varies its length so much as 10 μm. Accordingly, in aportion incapable of taking a large volume or a cross-sectionstructurally, the stiffness becomes low, and its position is easilydisplaced by the back force component and the cutting force generated inmachining. That is, in a multi-axis machining apparatus, there is aproblem that if it is made up of a material based on conventional castiron, the lowering of stiffness necessarily occurs caused by the elasticdeformation of not only the mechanism portions but also the materialitself.

In contrast with it, as this structure, if the Young's modulus of thematerial of at least one of the above-mentioned first workbench, theabove-mentioned second workbench, the above-mentioned static-pressureslide, and the bases for fixing them is made to be 200 GPa or greater,which is equivalent to two times or greater of the Young's modulus of aconventional material; therefore, it is possible to reduce by half thedisplacement of the above-mentioned member of the length 10 cm.

A machining apparatus set forth in Item (21) is a machining apparatus asset forth in any one of the Items (5) to (9) and (14) to (20), whereinthe pressure transmission medium of the aforesaid static-pressure slideis a liquid, and its viscosity is 10 pois or less.

Generally speaking, in order to make high the accuracy of the operationof an axis member, it is necessary to weaken the frictional force of theslide; therefore, in a high-accuracy machining apparatus, usually astatic-pressure slide is used. However, if the speed of the movement ofthe workbench is made high, there has been a problem that the pressuretransmission medium jetted into the clearance of the static-pressureslide generates heat by the shearing force caused by its viscosityresistance, and the workbench is warmed from the static-pressure surfaceto be thermally expanded; owing to this, the position of a tool or theposition of a work piece is displaced, which prevents a high-accuracymachining.

One of the most effective methods of preventing this is to lower theviscosity of the pressure transmission medium to be used in thestatic-pressure slide. By the practice of this, not only the viscosityresistance of the pressure transmission medium flowing in thestatic-pressure clearance is reduced and heat generation is suppressed,but also the pressure loss is reduced; therefore, the supply pressureacts on the static-pressure surface without being lowered, and thestiffness can be made larger. That is, to lower the viscosity of thepressure transmission medium has an effect for two important factorsrequired for a high-accuracy machining, which are the suppression ofheat generation and the strengthening of the stiffness. Further, if thespeed of the axis member is made higher than the speed of the flow ofthe pressure transmission medium in the clearance of the static-pressureslide, it has been generated a phenomenon that the pressure transmissionmedium cannot comply with it, and does not prevail over the wholestatic-pressure surface. This phenomenon lowers the stiffness of thestatic-pressure slide sharply, and generates a vibration owing to theunstable supporting; therefore, it has been a cause for axis members tobe prevented from a high-speed driving. In order to prevent this, it isconsidered to lower the viscosity of the pressure transmission medium,and by the practice of this, the shearing frictional force of thepressure transmission medium is weakened, which makes it possible tomake the pressure transmission medium prevail over the whole surface ofthe static-pressure clearance surface even in a high-speed driving. Asthe result, a high-speed driving of an axis member becomes possible.

As regards the pressure transmission medium, a liquid can make thestiffness higher than a gas for the same supply pressure, and on top ofit, a liquid has a better damping characteristic; therefore, a liquid isdesirable because it hardly generates a vibration even when its pressureis made high, and it is insensitive even to a suddenly changing externalforce. Accordingly, so long as the pressure transmission medium is aliquid, on the basis of the sealing and the supply pressure of a pumpfor supplying it, it is desirable that its viscosity is 10 pois or less,while its lower limit is of an order of 1 pois, which is close to theviscosity of water.

Actually, for a workbench which gave a stiffness of 1000 N/μm at asupply pressure of 20 atm. pressure when the clearance of thestatic-pressure slide was made to be 10 μm and an oil having a viscosityof 30 pois was employed for the pressure transmission medium, in thecase where an oil having a viscosity of 2 pois was employed through thechange of the orifice diameter of the static-pressure pad, a stiffnessof 1200 N/μm was obtained at a supply pressure of 5 atm. pressure. Ontop of it, the temperature of the workbench rose by almost 1° C. owingto the oil supply in the former case, but in the latter case, thetemperature rose by only 0.1° C., exhibiting only a very small change.

A machining apparatus set forth in Item (22) is a machining apparatus asset forth in any one of the Items (1) to (21), further comprising anactive control means for suppressing the transmission of a vibrationfrom the floor on which said machining apparatus is installed to saidmachining apparatus. In the above statement, the term “an active controlmeans” signifies a mean such that it comprises a measuring element fordetecting a displacement, velocity, acceleration, etc. of the floor asthe source of vibration, comprises a mechanism for minutely driving themount, and practices the removal of the vibration, that is, thesuppression or interception of the transmission of the vibration to themachining apparatus, by making the mount vibrate in such a manner as tocancel the vibration on the basis of the output of the measuringelement.

Among machining apparatuss on the market having a degree of freedom oftwo or three axes, some of them actualizes a high-accuracy machining byremoving minute vibrations from the floor with its main body carried ona air spring called an air mount. However, this air mount iscomparatively effective against vibrations with a frequency of 10 Hz orhigher, to be able to carry out the removal of vibrations; however,because it always has a resonance point at several Hz, vibrations of alow frequency are transmitted from the floor to the main body of themachining apparatus. In some case, a vibration of the floor is amplifiedand transmitted to the main body of the machining apparatus.Accordingly, an effective removal of floor vibrations is indispensableto a high-accuracy machining; in particular, as regards a machiningapparatus having a degree of freedom of three axes or greater, becauseits axis members are piled up in two or three stages to make its centerof gravity high and are fixed to their bases at their bottom surfaces,it has a structure such that it is easier to generate a moment-likeoscillation than a conventional machining apparatus having a degree offreedom of two axes or less. That is, a machining apparatus having adegree of freedom of three axes or greater has a tendency to have aresonance frequency for a minute vibration which is low for the reasonof its structure. Accordingly, it is very disadvantageous that aconventional air mount has a resonance point at a low frequency, andthere has been a great difficulty in the practice of a high-accuracymachining with a high-accuracy machining apparatus having a degree offreedom of three axes or greater supported by such a passive air mount.However, heretofore, because no consideration has been taken from such aviewpoint, it has never been used, an active air mount which practicesremoval of vibrations actively even against a low-frequency vibration.

It is desirable in a machining apparatus of a degree of freedom of threeaxes or greater that, by means of an active air mount having noresonance point even at a low frequency, the vibrations of the floor areremoved with a good efficiency, which makes the machining apparatusexhibit a stable accuracy of machining. Especially, the method ofcontrol of an active air mount is important. As regards a conventionalmachining apparatus having a degree of freedom of two axes or less,because each of the axis members is individually fixed to the base to belocated in a horizontal plane, if an active control is practiced for arotary vibration around any one of the two axes in the horizontaldirection or a vertical axis, the control is very satisfactory for theresult of machining, and reversely, there is almost no effect of theactive control for a parallel vibration in the vertical direction; incontrast with this, as regards a machining apparatus having a degree offreedom of three axes or greater, because it has a slide to be driven inthe vertical direction, an active control is effective for a parallelvibration in this direction also, and the practice of an active controlis effective for all the six axes of the degree of freedom including thetwo rotation axes parallel to a horizontal axis. In this way, it can besaid that it is important that a control for the removal of vibrationsis practiced with the driving direction of the axis members and theircharacteristics taken into consideration sufficiently, and theparameters of vibration suppression are individually optimized.

A machining apparatus set forth in Item (23) is a machining apparatus ofa degree of freedom of three axes or greater comprising a firstworkbench made of a material having a specific weight of 4 g/cm³ movedalong a static-pressure slide by a driving means with a frequency of 50Hz (desirably 100 Hz) or higher where its servo-gain becomes −3 dB, anda measuring means with a resolution of 10 nm or less for measuring theposition of said first workbench.

A machining apparatus set forth in Item (24) is a machining apparatus asset forth in the Item (23), further comprising a second workbench madeof a material having a specific weight of 4 g/cm³ or less swivelingalong a static-pressure slide, and a measuring means having a resolutionof 1 angular second or less for measuring the angle of said secondworkbench.

A machining apparatus set forth in Item (25) is a machining apparatus asset forth in the Item (23) or (24), wherein the machining speed is 600mm/min or higher.

A machining apparatus set forth in Item (26) is a machining apparatus asset forth in any one of the Items (23) to (25), wherein at least one ofthe aforesaid first workbench, the aforesaid second workbench, theaforesaid static-pressure slide, and the bases for fixing them is formedof a material having a coefficient of linear expansion of 5×10⁻⁶ K⁻¹ orless.

A machining apparatus set forth in Item (27) is a machining apparatus asset forth in any one of the Items (23) to (26), wherein at least one ofthe aforesaid first workbench, aforesaid second workbench, the aforesaidstatic-pressure slide, and the bases for fixing them is formed of amaterial having a Young's modulus of 200 GPa or greater.

A machining apparatus set forth in Item (28) is a machining apparatus asset forth in any one of the Items (23) to (27), wherein the pressuretransmission medium of the aforesaid static-pressure slide is a liquid,and its viscosity is 10 pois or lower.

A machining apparatus set forth in Item (29) is a machining apparatus asset forth in any one of the Items (23) to (28), further comprising anactive suppressing means for suppressing the transmission of a vibrationfrom the floor on which said machining apparatus is installed to saidmachining apparatus.

A machining apparatus set forth in Item (30) is a machining apparatus asset forth in any one of the Items (23) to (29), wherein at least one ofthe aforesaid first workbench, aforesaid second workbench, the aforesaidstatic-pressure slide, and the bases for fixing them is formed of amaterial containing a silicon nitride component of 50% by weight asconverted into Si₃N₄ or more.

By this invention, it is possible to provide a machining apparatus of adegree of freedom of three axes or greater to make possible thecompatibility of its high accuracy of machining with its high efficiencyof machining.

In the following, the embodiment of this invention will be explained indetail with reference to the drawings. FIG. 1 is a perspective view of a5-axis machining apparatus 10 of this embodiment. In FIG. 1, an activeair mount 11 supported on a floor F by four legs 11 a (only three legsare shown) is a suppression means for suppressing the transmission ofvibrations and has a function not to transmit a vibration of the floorto the base 12.

On a rail 12 a of the base 12 supported on the active air mount 11,there is provided a slide table 13 movably in the Z-axis direction, andon the slide table 13, there is provided a swivel table 14 rotatably. Inaddition, the slide table 13 and the swivel table 14 are supported in alow-friction manner by their respective static-pressure slides (notshown in the drawing) with a liquid introduced as a medium with respectto the rail 12 a and the slide table 13 respectively.

Further, on the base 12, at a rail 15 a laid over a pair of supportblocks 15, there is provided a slide table 16 movably in the X-axisdirection, at the rail 16 a on the slide table 16, there is provided aslide table 17 movably in the Y-axis direction, and on the slide table17, there is provided a swivel table 18 rotatably. In addition, theslide table 16, the slide table 17, the swivel table 18 are supported ina low-friction manner by their respective static-pressure slides (notshown in the drawing) with an oil introduced as a medium with respect tothe rail 15 a, the rail 16 a, and the slide table 17 respectively.

In this embodiment, for the slide tables 13, 16, and 17, which areregarded as the first workbench, laser scales having a measuringresolution of 1 nm are provided, which makes it possible to measuretheir travels, and it is actualized to carry out a driving by alinear-motor as the driving means with a frequency of 50 Hz (desirably100 Hz) or higher where the servo-gain becomes −3 dB. On the other hand,on the swivel tables 14 and 18, there are installed rotary encodershaving an angular resolution of 0.1 angular second, which makes itpossible to measure the rotational angle. In the static-pressure slideof this embodiment, the viscosity of the oil was made 2 pois, and thesupply pressure was made 5 atm. pressure. In this case, the stiffness ofthe slide shaft in the horizontal/vertical direction was 1350 N/μm,which was a sufficient value.

The slide tables 13, 16, and 17, the swivel tables 14 and 18, and thestatic-pressure slides were all made of silicon nitride, and as regardsthe swivel axis, its rotor portion was made of silicon nitride, and itsstator portion was made of a special alloy having a coefficient oflinear expansion of 4×10⁻⁶. Further, for the support blocks 15, invarwas used. For the base 12, a special alloy having a coefficient oflinear expansion of 4×10⁻⁶ K⁻¹ is used by welding. The Young's modulusof this special alloy was 130 GPa, and by the thickness of the platebeing made to be 40 mm, a stiffness necessary for a base was secured.

A work piece for generating a free curved optical surface was fitted tothe swivel table 14, a diamond tool (not shown in the drawing) was setto the swivel table 18, and by a simultaneous operation of the slidetables 13, 16, and 17 and the swivel table 14 (4-axis machining), acutting machining for shaping was carried out. The feed rate of theslide table 13 was 600 mm/min or higher, and the machining time was 36hours. The surface roughness of the cut surface was 5 nm by Rmax, andthe accuracy of shape was 57 nm; thus, a high accuracy about 3 times theaccuracy of machining of a multi-axis machining apparatus on the marketwas achieved.

FIG. 2 is a cross-sectional view of a slide table and a swivel table ina modified example of this embodiment (equivalent to a cross-sectionalview at the II-II line of FIG. 1). In FIG. 2, on a rail support member12 a of a base 12, a flat-plate-shaped rail 12 b which is formed of aceramic material and extending to the direction perpendicular to thepaper surface is fixed. In such a way as to cover the rail 12 b, a slidetable 113 formed of a ceramic material having a U-shaped cross-sectionis arranged.

The slide table 113 forms static-pressure pads 113 a and 113 a (a thinspace or a porous material each) at the downward facing portion of itsinner peripheral surface opposite to the upper surface of the rail 12 b,forms static-pressure pads 113 b and 113 b at the portion of its innerperipheral surface opposite to the side surface of the rail 12 b, andforms static-pressure pads 113 c and 113 c at the upward facing portionof its inner peripheral surface opposite to the lower surface of therail 12 b. To each of the static-pressure pads 113 a to 113 c, oil of aspecified pressure is supplied through holes 113 d which are present asextending in the slide table 113. In addition, to the slide table 113,an encoder (not shown in the drawing) is fixed, and on the other hand,opposite to this, a sensor (not shown in the drawing) is provided on thebase 12; thus, it is actualized to make it possible to measure thetravel of the slide table 113 with respect to the base 12 with aresolution of 10 nm or less. The encoder and the sensor makes up ameasuring means.

On the upper surface of the slide table 113, a support table 120 isfixed. It is desirable that the support table is made of an alloy suchas invar for the purpose of burying a coil etc. therein as will bedescribed later, but it is also possible to use a ceramic material if itis workable.

The support table 120 having approximately a shape of a hollow cylindercontains inside a swivel table unit 114. To be more concrete, the swiveltable unit 114 has a shape such that the lower gear-shaped portion 114 aformed of a magnetic material and an upper disk portion 114 b formed ofa ceramic material are coupled through a disk-shaped reduced-diameterportion 114 f. The gear-shaped portion 114 a has a plurality of teethformed on its outer circumference, and has N poles and S polesalternately arranged by the magnetizing of the teeth one by one.Opposite to these teeth, on the inner peripheral surface of the supporttable 120, there are arranged coils C of a number larger by one than thenumber of the teeth of the gear-shape portion 114 a. The gear-shapedportion 114 a and the coils C makes up a servomotor.

On the lower surface of the gear-shaped portion 114 a, an encoder 114 cis fixed; on the other hand, opposite to this, a sensor 114 d isprovided on the support table 120, which makes it possible to measurethe rotational angle of the swivel table unit 114 with respect to thesupport table 120 with a resolution of 1 angular second or less. Theencoder 114 c and the sensor 114 d makes up a measuring means.

The support table 120 forms a ring-shaped static-pressure pad 120 a onthe lower surface of its upper flange 120 f opposite to the uppersurface of gear-shaped portion 114 a, forms a ring-shapedstatic-pressure pad 120 b on the inner peripheral surface of its upperflange 120 f opposite to the outer peripheral surface of thereduced-diameter portion 114 f of the swivel table unit, and forms aring-shaped static-pressure pad 120 c on the upper surface of its lowerflange 120 e opposite to the lower surface of the gear-shaped portion114 a. To each of the static-pressure pads 120 a to 120 c, oil of aspecified pressure is supplied through a hole 120 d which is provided asextending in the support table 120.

The operation of this embodiment will be explained. By the supply of oilfrom an external oil pressure source to the hole 113 d, oil is jettedfrom the static-pressure pads 113 a to 113 c, and by the use of thestatic pressure, the slide table 113 carrying the support table 120 issupported against the rail 12 b in a state of non-contact with it. Inthis state, by the driving of a linear motor (not shown in the drawing),the slide table 113 is to be moved to a desired position with respect tothe base 12.

Further, by the supply of oil from an external oil pressure source tothe hole 120 d, oil is jetted from the static-pressure pads 120 a to 120c, and by the use of the static pressure, the swivel table unit 114supporting a work piece (not shown in the drawing) is supported againstthe support table 120 in a state of non-contact with it. In this state,by the application of an alternating current to the coils C, thegear-shaped portion 114 a is magnetically driven, and the swivel tableunit 114 is to be rotated by a desired angle with respect to the supporttable 120.

By this embodiment, because the slide table 113 as the first workbenchis formed of a ceramic material, even in a case where a temperaturevariation is produced, thermal expansion can be controlled to be smalleras compared to steel; therefore, the accuracy of machining can bemaintained while the acceleration/deceleration performance is madehigher due to the slide table being capable of weight reduction, whichmakes it possible to shorten the machining time. In particular, becausethe rail 12 b is formed of a ceramic material having the samecoefficient of linear expansion as that of the slide table 113, in acase where a temperature change is produced, equal thermal expansionsare produced; therefore, it is possible to suppress the change of thestatic-pressure clearance at the minimum, to carry out a high-accuracymachining. Further, because it is enough even if the capacity of thelinear motor or the like as the driving means for driving the slidetable 113 is small, energy saving can be achieved while the quantity ofheat generation can also be suppressed; therefore, a higher-accuracymachining can be actualized.

FIG. 3 is a cross-sectional view similar to FIG. 2 showing anothermodified example of this embodiment. Rails made of a ceramic material 12b and 12 b placed on a base 12 are separately positioned at the left andright in the drawing, a slide table 213 made of a ceramic material asthe first workbench forms a projection portion 213 f extending in thelengthwise direction between the rails 212 b and 212 b, of which on theside surface static-pressure pads 213 b and 213 b are provided. By doingthis way, the bending of the slide table 213 can be removed, and thestiffness for support can be made higher.

Further, as regards a swivel table unit 214 made of a ceramic materialas the second workbench, its reduced-diameter portion 214 f has adouble-tapered shape with its diameter more reduced gradually towardsits center in the vertical direction. Further, as regards an upperflange 220 f provided as extending from the support table 220, its innerperiphery in the radial direction has a tapered shape corresponding tothe reduced-diameter portion 214 f of the swivel table unit 214, and onthe upper and lower tilt surfaces of the inner periphery, there areprovided ring-shaped static-pressure pads 220 a and 220 b. As regardsthe structure of other portions, explanation will be omitted becausethey are the same as those in the modified example shown in FIG. 2.

FIG. 4 is a cross-sectional view similar to FIG. 2 showing furtheranother modified example of this embodiment, but its support base andswivel table unit are omitted. In FIG. 4, a rail 312 b made of a ceramicmaterial placed on the base 12 has a cross-section of a shape of areverse trapezoid, and a slide table 313 also made of a ceramic materialhas an inner peripheral surface of a shape corresponding to theabove-mentioned cross-section.

The slide table 313 as the first workbench forms a static-pressure pad313 a on the inner peripheral surface opposite to the upper surface ofthe rail 312 b, forms static-pressure pads 313 b and 313 b on the innerperipheral tilt surface opposite to the side tilt surface of the rail312 b, and forms static-pressure pads 313 c and 313 c on its bottomsurface opposite to the top surface of a base 12. It is to be done thatto each of the static-pressure pads 313 a to 313 c, oil of a specifiedpressure is supplied from the outside through holes 313 c provided asextending in the slide table 313. As regards the structure of otherportions, explanation will be omitted because they are the same as thosein the modified example shown in FIG. 2.

Up to now, this invention has been explained with reference to theembodiment; however, it is a matter of course that this invention shouldnot be construed with a limitation to the above-mentioned embodiment,and can be suitably modified or altered. For example, even if at leastone workbench that is driven at the highest speed of all is formed of aceramic material, the effect of this invention can be exhibited.

1. A machining apparatus for conducting at least one of a cuttingprocess and a grinding process, comprising: a stationary base; and aworking device mounted on the stationary base and having a degree offreedom of more than two axes to conduct at least one of a cuttingprocess and a grinding process, the working device comprising a firstworking bench which is formed of a ceramic material and is movablelinearly with a degree of freedom of not less than one axes or morewhile holding a work piece or a tool.
 2. The machining apparatus ofclaim 1, wherein the ceramic material has a coefficient of linearexpansion of not more than 5×10⁻⁶ K⁻¹.
 3. The machining apparatus ofclaim 1, wherein the ceramic material contains silicon nitride of notless than 50% by weight as converted into Si₃N₄ and has a specificweight of not more than 4 g/cm³.
 4. The machining apparatus of claim 1,wherein the ceramic material has a Young's modulus of not less than 200GPa.
 5. The machining apparatus of claim 1, wherein the first workingbench is driven along a static-pressure guide by an axis driving devicewith a frequency of not less than 50 Hz with a servo gain of −3 dB. 6.The machining apparatus of claim 5, wherein the axis driving device is alinear motor.
 7. The machining apparatus of claim 1, wherein the workingdevice further comprises a measurement device having a resolution of notmore than 10 nm for measuring the position of the first working bench.8. The machining apparatus of claim 1, wherein a machining speed for awork piece or a tool held by the first working bench is not less than600 mm/min.
 9. The machining apparatus of claim 8, wherein the firstworking bench is driven at the highest speed in the machining apparatus.10. A machining apparatus for conducting at least one of a cuttingprocess and a grinding process, comprising: a stationary base; and aworking device mounted on the stationary base and having a degree offreedom of more than two axes to conduct at least one of a cuttingprocess and a grinding process, the working device comprising a secondworking bench which is formed of a ceramic material and is rotatablewith a degree of freedom of not less than one axes while holding a workpiece or a tool.
 11. The machining apparatus of claim 10, wherein theceramic material has a coefficient of linear expansion of not more than5×10⁻⁶ K⁻¹.
 12. The machining apparatus of claim 10, wherein the ceramicmaterial contains silicon nitride of not less than 50% by weight asconverted into Si₃N₄ and has a specific weight of not more than 4 g/cm³.13. The machining apparatus of claim 10, wherein the ceramic materialhas a Young's modulus of not less than 200 GPa.
 14. The machiningapparatus of claim 10, wherein the second working bench is driven alonga static-pressure guide by an axis driving device with a frequency ofnot less than 50 Hz with a servo gain of −3 dB.
 15. The machiningapparatus of claim 14, wherein the axis driving device is an ACservomotor.
 16. The machining apparatus of claim 10, wherein the workingdevice further comprises a measuring device having a resolution of notmore than a 1 angular second for measuring the angle of the secondworking bench.
 17. The machining apparatus of claim 10, wherein arotational machining speed for a work piece or a tool held by the secondworking bench is not less than 1°/sec.
 18. The machining apparatus ofclaim 17, wherein the second working bench is driven at the highestspeed in the machining apparatus.
 19. The machining apparatus of claim10, wherein the working device further comprises a support table forsupporting the second working bench and the support table is formed ofat least one of a ceramic material satisfying at least one of conditionsof having a coefficient of linear expansion of not more than 5×10⁻⁶ K⁻¹,containing silicon nitride of not less than 50% by weight as convertedinto Si₃N₄ and having a specific weight of not more than 4 g/cm³, andhaving a Young's modulus of not less than 200 GPa, and an alloycontaining nickel from 10% by weight to 50% by weight.
 20. The machiningapparatus of claim 5, wherein at least one of the static-pressure guideand a base for fixing it is formed of a ceramic material satisfying atleast one of the conditions of having a coefficient of linear expansionof not more than 5×10⁻⁶ K⁻¹, containing silicon nitride of not less than50% by weight as converted into Si₃N₄ and having a specific weight ofnot more than 4 g/cm³, and having a Young's modulus of not less than 200GPa.
 21. The machining apparatus of claim 14, wherein at least one ofthe static-pressure guide and a base for fixing it is formed of aceramic material satisfying at least one of the conditions of having acoefficient of linear expansion of not more than 5×10⁻⁶ K⁻¹, containingsilicon nitride of not less than 50% by weight as converted into Si₃N₄and having a specific weight of not more than 4 g/cm³, and having aYoung's modulus of not less than 200 GPa.
 22. The machining apparatus ofclaim 5, wherein a pressure transmission medium of the static-pressureguide is a liquid having a viscosity of not more than 10 pois.
 23. Themachining apparatus of claim 14, wherein a pressure transmission mediumof the static-pressure guide is a liquid having a viscosity of not morethan 10 pois.
 24. The machining apparatus of claim 1, wherein theworking device further comprises an active control device forsuppressing the transmission of a vibration from the floor on which themachining apparatus is installed to the machining apparatus.
 25. Themachining apparatus of claim 10, wherein the working device furthercomprises an active control device for suppressing the transmission of avibration from a floor on which the machining apparatus is installed tothe machining apparatus.
 26. A machining apparatus for conducting atleast one of a cutting process and a grinding process, comprising: astationary base; and a working device mounted on the stationary base andhaving a degree of freedom of more than two axes to conduct at least oneof a cutting process and a grinding process, the working devicecomprising a first working bench which is made of a material having aspecific weight of not more than 4 g/cm³ and is moved along a firststatic-pressure guide by a driving device with a frequency of not lessthan 50 Hz with a servo-gain of −3 dB and a measuring device with aresolution of not more than 10 nm for measuring the position of thefirst working bench.
 27. The machining apparatus of claim 26, whereinthe working device further comprises a second working bench made of amaterial having a specific weight of not more than 4 g/cm³ swivelingalong a second static-pressure guide and a measuring device having aresolution of not more than 1 angular second for measuring the angle ofthe second working bench.
 28. The machining apparatus of claim 26,wherein a machining speed is not less than 600 mm/min.
 29. The machiningapparatus of claim 27, wherein at least one of the first working bench,the second working bench, the first static-pressure guide, the secondstatic-pressure guide, and a base for fixing them is formed of amaterial having a coefficient of linear expansion of not more than5×10⁻⁶ K⁻¹.
 30. The machining apparatus of claim 27, wherein at leastone of the first working bench, the second working bench, the firststatic-pressure guide, the second static-pressure guide, and a base forfixing them is formed of a material having a Young's modulus of not lessthan 200 Gpa.
 31. The machining apparatus of claim 26, wherein apressure transmission medium of the first static-pressure guide is aliquid having a viscosity of not more than 10 pois.
 32. The machiningapparatus of claim 26, the working device further comprises an activesuppressing device for vibration from a floor on which the machiningapparatus is installed to the machining apparatus.
 33. The machiningapparatus of claim 27, wherein at least one of the first working bench,the second working bench, the first static-pressure guide, the secondstatic-pressure guide and a base for fixing them is formed of a materialcontaining a silicon nitride component of not less than 50% by weight asconverted into Si₃N₄.